Method of fabricating a storage gate pixel design

ABSTRACT

A method of fabricating a pixel cell having a shutter gate structure. First and second charge barriers are respectively created between a photodiode and a first charge storage region and between the first storage region and a floating diffusion region. A global shutter gate is formed to control the charge barrier and transfer charges from the photodiode to the first charge storage region by effectively lowering the first charge barrier. A transfer transistor acts to transfer charges from the first storage region to the floating diffusion region by reducing the second charge barrier.

FIELD OF THE INVENTION

The present invention relates to the field of semiconductor devices and,in particular, to a method of fabricating a pixel that includes astorage gate.

BACKGROUND OF THE INVENTION

A CMOS imager includes a focal plane array of pixel cells, each cellincludes a photosensor, for example, a photogate, photoconductor or aphotodiode overlying a substrate for producing a photo-generated chargein a doped region of the substrate. In a CMOS imager, the activeelements of a pixel cell, for example a four transistor (4T) pixel,perform the necessary functions of (1) photon to charge conversion; (2)transfer of charge to the floating diffusion region; (3) resetting thefloating diffusion region to a known state before the transfer of chargeto it; (4) selection of a pixel cell for readout; and (5) output andamplification of a signal representing a reset voltage and a pixelsignal voltage based on the photo converted charges. The charge at thefloating diffusion region is converted to a pixel or reset outputvoltage by a source follower output transistor.

Exemplary CMOS imaging circuits, processing steps thereof, and detaileddescriptions of the functions of various CMOS elements of an imagingcircuit are described, for example, in U.S. Pat. No. 6,140,630, U.S.Pat. No. 6,376,868, U.S. Pat. No. 6,310,366, U.S. Pat. No. 6,326,652,U.S. Pat. No. 6,204,524, and U.S. Pat. No. 6,333,205, all assigned toMicron Technology, Inc. The disclosures of each of the forgoing patentsare hereby incorporated by reference herein in their entirety.

A schematic diagram of a conventional CMOS four-transistor (4T) pixelcell 10 is illustrated in FIGS. 1A and 1B. FIG. 1A is a top-down view ofthe cell 10; FIG. 1B is a cross-sectional view of the cell 10 of FIG.1A, taken along line A-A′. The illustrated cell 10 includes a pinnedphotodiode 13 as a photosensor. Alternatively, the CMOS cell 10 mayinclude a photogate, photoconductor or other photon-to-charge convertingdevice, in lieu of the pinned photodiode 13, as the initial accumulatingarea for photo-generated charge. The photodiode 13 includes a p+ surfaceaccumulation layer 5 and an underlying n− accumulation region 14 formedin a p-type semiconductor substrate layer 2.

The pixel cell 10 of FIG. 1 has a transfer gate 7 for transferringphotocharges generated in the n− accumulation region 14 to a floatingdiffusion region 3 (i.e., storage region). The floating diffusion region3 is further connected to a gate 27 of a source follower transistor. Thesource follower transistor provides an output signal to a row selectaccess transistor having a gate 37 for selectively gating the outputsignal to a terminal (not shown). A reset transistor having a gate 17resets the floating diffusion region 3 to a specified charge levelbefore each charge transfer from the n− region 14 of the photodiode 13.

The illustrated pinned photodiode 13 is formed on a p-type substrate 2.It is also possible, for example, to have a p-type substrate base in ann-type epitaxial layer. The n− accumulation region 14 and p+accumulationregion 5 of the photodiode 13 are spaced between an isolation region 9and a charge transfer gate 7. The illustrated, conventional pinnedphotodiode 13 has a p+/n−/p− structure.

Imager pixels, including CMOS imager pixels 10 typically have low signalto noise ratios and narrow dynamic range because of their inability tofully collect, transfer and store the electric charge collected by thephotosensitive area of the photosensor. In addition, the pixels aresubject to kTC noise, which is a thermal dependent noise generatedduring the reset of the pixel. The kTC noise refers to the randomvariation of voltage during the reset of a diffusion area or a storagecapacitor.

Because the size of the pixel electrical signal is very small, thesignal to noise ratio and dynamic range of the pixel should be as highas possible. In addition, customer demands increasingly call forapplications requiring higher dynamic range. The use of additional gatesto increase the functional operations of the pixel (e.g., electronicshuttering), however, increases the size of the pixel or reduces thefill factor of the pixel.

One invention that has been suggested for dealing with noise in scaledpixels while providing an electronic shutter is a shutter gate. When ashutter gate is implemented in a pixel design, a storage node is alsoadded such that charges accumulated in a photodiode 13 are transferredthrough the shutter gate to a storage node. The additional storage nodeallows the floating diffusion node to be reset and readout prior tocharge transfer to the floating diffusion node, thus allowing forcorrelated double sampling and a reduction of kTC noise. The amount ofcharge the pixel can store also increases since the gated storage nodehas a greater charge storage capacity than the photodiode node. Anexample of a pixel incorporating a shutter gate is U.S. application Ser.No. 10/721,191, assigned to Micron Technology Inc., and incorporatedherein by reference.

Additionally, in conventional pixel cells, potential barriers may existin the path of the photo-generated charge as it is transferred from thephoto-conversion device to readout circuitry. Such potential barriersmay prevent a portion of the photo-generated charge from reaching thereadout circuitry, thereby reducing the charge transfer efficiency ofthe pixel cell and also reducing the quality of a resultant image.Accordingly, what is needed is a relatively simple method forfabricating a pixel cell with an electrical shutter having good chargetransfer characteristics with low charge loss.

BRIEF SUMMARY OF THE INVENTION

Exemplary embodiments of the invention provide a method of fabricating apixel cell having a global shutter gate structure in which a first gatedcharge barrier is provided between a photosensor and a first chargestorage region and a gated second charge barrier is provided between thefirst charge storage region and a floating diffusion region. A globalshutter gate controls the first charge barrier while a transfer gatecontrols the second charge barrier.

In accordance with one modified exemplary embodiment of the invention, acapacitive structure is also formed over the pixel sensor cell in orderto provide overall increased charge storage capacity.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages and features of the invention willbecome more apparent from the detailed description of exemplaryembodiments provided below with reference to the accompanying drawingsin which:

FIG. 1A is a top-down view of a conventional pixel sensor cell;

FIG. 1B is a cross sectional view of the conventional pixel sensor cellof claim 1A taken along line A-A′;

FIG. 2A is a top-down view of an exemplary pixel sensor cell constructedin accordance with the present invention;

FIG. 2B is a cross-sectional view of the exemplary pixel sensor cell ofFIG. 2A, taken along line B-B′;

FIG. 3A is a cross-sectional view of the exemplary pixel sensor cell ofFIG. 2A taken at an initial stage of fabrication in accordance with afirst exemplary embodiment;

FIG. 3B is a cross-sectional view of the exemplary pixel sensor cell ofFIG. 2A taken at an initial stage of fabrication in accordance with asecond exemplary embodiment;

FIG. 4 is a cross-sectional view of the exemplary pixel sensor cell ofFIG. 2A taken a stage of fabrication subsequent to that shown in FIGS.3A and 3B;

FIG. 5 is a cross-sectional view of the exemplary pixel sensor cell ofFIG. 2A taken a stage of fabrication subsequent to that shown in FIG. 4;

FIG. 6 is a cross-sectional view of the exemplary pixel sensor cell ofFIG. 2A taken a stage of fabrication subsequent to that shown in FIG. 5;

FIG. 7 is a cross-sectional view of the exemplary pixel sensor cell ofFIG. 2A taken a stage of fabrication subsequent to that shown in FIG. 6;

FIG. 8 is a cross-sectional view of the exemplary pixel sensor cell ofFIG. 2A taken a stage of fabrication subsequent to that shown in FIG. 7;

FIG. 9 is a cross-sectional view of the exemplary pixel sensor cell ofFIG. 2A taken a stage of fabrication subsequent to that shown in FIG. 8;

FIG. 10 is a cross-sectional view of the exemplary pixel sensor cell ofFIG. 2A taken a stage of fabrication subsequent to that shown in FIG. 9;

FIG. 11 is a cross-sectional view of the exemplary pixel sensor cell ofFIG. 2A taken a stage of fabrication subsequent to that shown in FIG.10;

FIG. 12 is a cross-sectional view of the exemplary pixel sensor cell ofFIG. 2A taken a stage of fabrication subsequent to that shown in FIG.11;

FIG. 13 is a cross-sectional view of the exemplary pixel sensor cell ofFIG. 2A taken a stage of fabrication subsequent to that shown in FIG.12;

FIG. 14 depicts a block diagram of an imaging device including anexemplary pixel sensor cell constructed in accordance with theinvention; and

FIG. 15 shows a processor system incorporating at least one imagerdevice constructed in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and in which is shown byway of illustration specific embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that other embodiments may be utilized, and thatstructural, logical and electrical changes may be made without departingfrom the spirit and scope of the present invention.

The term “substrate” is to be understood as a semiconductor-basedmaterial including silicon, silicon-on-insulator (SOI) orsilicon-on-sapphire (SOS) technology, doped and undoped semiconductors,epitaxial layers of silicon supported by a base semiconductorfoundation, and other semiconductor structures. Furthermore, whenreference is made to a “substrate” in the following description,previous process steps may have been utilized to form regions orjunctions in the base semiconductor structure or foundation. Inaddition, the semiconductor need not be silicon-based, but could bebased on silicon-germanium, germanium, or gallium arsenide.

The term “pixel” refers to a picture element unit cell containing aphotosensor and transistors for converting light radiation to anelectrical signal. For purposes of illustration, a representative pixelis illustrated in the figures and description herein and, typically,fabrication of all pixels in an imager will proceed simultaneously in asimilar fashion.

Although the invention is described herein with reference to thearchitecture and fabrication of one pixel cell, it should be understoodthat this is representative of a plurality of pixel cells in an array ofan imager device such as array 240 of imager device 308 (FIG. 14). Now,referring to the drawings, where like numbers represent like elements,FIGS. 2A and 2B show a top-down and cross sectional view, respectively,of exemplary pixel cell 100. Pixel cell 100 has doped regions whichcreate charge barriers both used to block and efficiently move chargesfrom a photosensor, e.g. a photodiode 124, through a shutter transistor110, to a storage area 114, and from there through a transfer transistor130 to a floating diffusion region 134 which provides a pixel signal tothe gate of a source follower transistor 137 for output.

Pixel cell 100 is formed in a p-type substrate 101. The pixel cell 100has three large p-wells in the substrate 101, as shown in FIG. 2B. Twodoped regions 104, 124 form a photodiode structure. The surface dopedregion 124 is illustratively doped p-type. The second doped region 104,located beneath the first doped region 124, is illustratively n-type.The second doped region is a charge accumulation region 104. These twodoped regions 104, 124 create a p/n/p photodiode in the substrate 101.Located on either side of the accumulation region 104 are p-type dopedregions 150, 151. Doped region 151 creates a controlled charge barrierbetween, for example, the accumulation region 104 and a first chargestorage region 114 and mitigates leakage between these regions. Dopedregion 150 mitigates leakage at the isolation region 109 edge.

The charge storage region 114 is formed in the substrate 101 at leastpartially inside of a p-well 161. Located partially over the firstcharge storage region 114 is a charge storage gate 110. In operation,the charge storage gate 110 transfers charges from the accumulationregion 104 of the photodiode to the charge storage region 114 bylowering a charge barrier between the two regions 104, 114. It should beunderstood that when pixel cell 100 is incorporated into a pixel array240 (FIG. 14) each of the shutter storage gates 110 must be electricallyconnected together by conductive wiring in order to turn on and off eachstorage gate 110 at the same time in order to perform a global shutter.

Next, a transfer transistor gatestack 130 is formed adjacent to thecharge storage gate 110. Like the charge storage gate 110, the transfertransistor gatestack 130 provides for the transfer of charges from thecharge storage region 114 to a floating diffusion region 134. Thefloating diffusion region 134 is a lightly doped n-type region formed atleast partially within a p-well 162 in the substrate 101. An additionalp-type doped region 152 is located between the charge storage region 114and the floating diffusion region 134. This p-type doped region 152provides a controlled charge barrier for the flow of charges betweenthese two regions 114, 134.

As shown schematically in FIG. 2B, the floating diffusion region 134 iselectrically connected to the gate of the source follower transistor 137for the readout operation. A reset transistor 127 is also connected tothe floating diffusion region 134, and is used to reset the charge ofthe diffusion region 134. The pixel cell 100 also has a row selecttransistor 147, which connects the output of source follower transistor137 to an associated column line 125 of a pixel array.

Pixel cell 100 also has shallow trench isolation regions 109 located oneither side of the cell 100. Each isolation region 109 is located withina p-well. Formed above the isolation regions 109 are capacitorstructures 119, which may be used to further increase the chargecapacity of the cell 100. It should be understood that the capacitivestructures 119 may also be formed at other locations on the pixel cell100, including locations that are electrically connected to either ofthe charge storage region 114 or the floating diffusion region 134.

In operation of the pixel cell 100 as part of an imager device 308 (FIG.14), the storage node 114 in the pixel cell 100 enables the floatingdiffusion region 134 to be reset and readout prior to chargetransference of photo-generated charge originating at the photodiodeaccumulation region 104 to the floating diffusion region 134 afterpassing through storage region 114. This also allows for double samplingof the pixel cell 100 and a reduction of kTC noise. The total amount ofcharge the pixel 100 can store is increased since the gated storage node114 has a greater charge storage capacity than the floating diffusionnode 134.

Additionally, the charge transfer efficiency for pixel cell 100 isimproved, as the storage gate 110 and transfer gate 130 effectivelycontrol the charge barriers between the charge accumulation region 104,charge storage area 114, and floating diffusion region 134 to ensurecomplete transfer of the charges generated in the photodiodeaccumulation region 104 which are then moved to the floating diffusionregion 134. Specifically, when the storage gate 110 is turned on bycontrol circuitry 250 (FIG. 14), the potential barrier between thecharge storage regions 104, 114 is lowered, as the p-type region 151 iseffectively inverted, thereby allowing accumulated electrons to flowfrom the accumulation region 104 to the charge storage region 114.Similarly, when the transfer gate 130 is turned on, the potentialbarrier between the charge storage regions 114, 134 is reduced, therebyallowing accumulated electrons to flow from the charge storage region114 to the floating diffusion region 134.

With reference to FIGS. 3A-13, a method of fabricating the pixel cell100 in accordance with a first exemplary embodiment is now described.During an initial stage of fabrication (FIG. 3A), isolation regions 109are formed in a semiconductor substrate 101. Shallow trench isolation isone technique that may be used to form isolation regions 109 althoughother techniques as known in the art may also be used. A sacrificialoxide layer 102 is formed at the top surface of the substrate 101. Theoxide layer 102 helps protect the substrate surface from damage andimpurities during the following processing. The sacrificial oxide layer102 may be formed by oxidizing the silicon substrate 101 surface, toform a layer of silicon dioxide 102, typically having a thickness in therange of about 50 to about 150 Angstroms thick.

Next, as shown in FIG. 4, a layer of photoresist is formed over thesacrificial oxide layer 102. The resist is patterned and developed asdesired to form openings where dopants will be implanted into thesubstrate 101. Next, using any suitable n-type dopants, n-type dopedregions 104 (charge accumulation region) and 114 (charge storage region)are formed. The n-type doped regions 104, 114 may have a dopantconcentration within the range of about 1e¹⁶ to about 1e¹⁸ atoms percm³, preferably within the range of about 5e¹⁶ to 5e¹⁷ atoms per cm³.The resist and sacrificial oxide layers are then stripped from thesubstrate 101 surface. Subsequently, a gate oxide layer 103 (FIG. 5) isformed over the substrate 101. The gate oxide layer 103 may be formed ofany suitable gate dielectric material.

As an alternative to these first two stages of fabrication, FIG. 3Billustrates an alternative method of fabricating the pixel cell 100 inaccordance with a second exemplary embodiment of the invention.Specifically, with reference to FIG. 3B, a sacrificial oxide layer 102is formed by oxidizing a top surface of the substrate 101, but thesacrificial oxide layer 102 is immediately removed after formation. Inits place, a gate oxide layer 103 is formed at the surface of thesubstrate 101. The gate oxide layer 103 may be formed of any suitablegate oxide material, including but not limited to silicon dioxide. Aftercompletion of this step, the remaining steps of fabrication areidentical for each of the methods. Thus, it should be understood thatthe remaining figures illustrate the remaining steps for fabricating apixel cell 100 having been initially fabricated in accordance witheither exemplary method just described.

Turning to FIG. 5, a layer of polysilicon 105 is deposited over the gateoxide layer 103. Next, as shown in FIG. 6, a blanket threshold voltage(Vt) adjustment implant deposits p-type dopants into the substrate 101.FIG. 7 illustrates a masked Vt adjustment implant which is accomplishedby a layer of resist being deposited and patterned to form selectiveopenings for the dopants. Any suitable p-type dopant may be utilized.This step is performed in order to adjust the threshold voltages of thesubsequently formed transistors and to create a controlled chargebarrier for the flow of electrons out of the accumulation region 104 andinto the first storage region 114 and the floating diffusion region 134(FIG. 2B). The p-type regions 150, 151, 152 formed are illustrated withdashed lines. These p-type regions 150, 151, 152 may have a p-typedopant concentration within the range of about 1e¹⁶ to about 1e¹⁸ atomsper cm³, preferably within the range of about 5e¹⁶ to 5e¹⁷ atoms percm³.

Next, with reference to FIG. 8, several layers are deposited over thesubstrate. An insulating layer 107 is formed over the polysilicon layer105. A second polysilicon layer 108 is then deposited over theinsulating layer 107. A tetraethyl orthosilicate, (TEOS)-based oxidelayer is formed over all of the layers 103, 105, 107, 108 to form aTEOS-based oxide cap. As shown in FIG. 9, an appropriate photoresist isutilized next and most of the insulating layer 107, second polysiliconlayer 108 and TEOS cap layer are selectively removed. In the area abovethe isolation regions 109, each of the layers 103, 105, 107, 109, TEOSremains, thereby forming a capacitive structure 119. Over the rest ofthe pixel cell 100, only the gate oxide 103 and first polysilicon layers105 remain.

Subsequently, the gate oxide 103 and polysilicon 105 layers are etchedto form gatestacks as shown in FIG. 10. Next, another photoresist layeris deposited and patterned to form openings for dopant implantation.Suitable p-type dopants are then implanted into the substrate 101 toform p-wells 160, 161, 162. The p-wells may have a concentration ofp-type dopants within the range of about 5e¹⁵ to about 1e¹⁸ atoms percm³, preferably within the range of about 1e¹⁶ to 1e¹⁷ atoms per cm³.Next, the photoresist layer is stripped, and an oxide layer 120 (FIG.11) is grown over the surface of the substrate 101. The oxide layer 120forms an insulating cap and sidewalls on each of the capacitivestructures 119 and surrounding the gatestacks 110, 130.

As shown in FIG. 12, a photoresist layer is formed and patterned suchthat it is present everywhere except over the area of the doped region104. In this area, p-type dopants are implanted into the substrate 101to form a p-type surface region 124. The dopant concentration forsurface region 124 may be within the range of about 2e¹⁷ to about 5e¹⁹atoms per cm³, and preferably within the range of about 5e¹⁷ to about5e¹⁸ atoms per cm³. The photoresist is then stripped, and another layerof photoresist is formed and patterned over the structures on the cell100 as shown in FIG. 13. N-type dopants are implanted through apatterned opening in the resist, in order to create floating diffusionregion 134 in the substrate 101. The floating diffusion region 134 has aconcentration of n-type dopants within the range of about 1e¹⁷ to about2e²⁰ atoms per cm³, preferably within the range of about 5e¹⁷ to 5e¹⁸atoms per cm³. The floating diffusion region 134 is located within ap-well 162.

At this stage, the formation of the exemplary pixel sensor cell 100 isessentially complete. Additional processing steps may be used to forminsulating, photo device shielding, and interconnect metallizationlayers as desired.

FIG. 14 illustrates a block diagram of an exemplary CMOS imager 308having a pixel array 240 comprising a plurality of pixels 100 arrangedin a predetermined number of columns and rows, with each pixel cellbeing constructed as in one of the illustrated embodiments describedabove. Attached to the array 240 is signal processing circuitry, asdescribed herein, at least part of which may be formed in the substrate.The pixels of each row in array 240 are all turned on at the same timeby a row select line, and the pixels of each column are selectivelyoutput by respective column select lines. A plurality of row and columnlines are provided for the entire array 240. The row lines areselectively activated by a row driver 245 in response to row addressdecoder 255. The column select lines are selectively activated by acolumn driver 260 in response to column address decoder 270. Thus, a rowand column address is provided for each pixel.

The CMOS imager is operated by the timing and control circuit 250, whichcontrols address decoders 255, 270 for selecting the appropriate row andcolumn lines for pixel readout. The control circuit 250 also controlsthe row and column driver circuitry 245, 260 such that they applydriving voltages to the drive transistors of the selected row and columnlines. The pixel column signals, which typically include a pixel resetsignal (V_(rst)) and a pixel image signal (V_(sig)), are read by asample and hold circuit 261. V_(rst) is read from a pixel 100immediately after the floating diffusion region 134 is reset by thereset gate 127. V_(sig) represents the amount of charges generated bythe photosensitive element of the pixel cell 100 in response to appliedlight. A differential signal (V_(rst)-V_(sig)) is produced bydifferential amplifier 262 for each pixel, which is digitized byanalog-to-digital converter 275 (ADC). The analog to digital converter275 supplies the digitized pixel signals to an image processor 280 whichforms and outputs a digital image.

FIG. 15 illustrates a processor-based system 1100 including an imagingdevice 308, which has pixels constructed in accordance with the methodsas described herein. For example, pixels may be the exemplary pixelcells 100 constructed in accordance with the exemplary embodiments ofthe invention described above. The processor-based system 1100 isexemplary of a system having digital circuits that could include imagesensor devices. Without being limiting, such a system could include acomputer system, camera system, scanner, machine vision, vehiclenavigation, video phone, surveillance system, auto focus system, startracker system, motion detection system, image stabilization system, anddata compression system.

The processor-based system 1100, for example a camera system, generallycomprises a central processing unit (CPU) 1102, such as amicroprocessor, that communicates with an input/output (I/O) device 1106over a bus 1104. Imaging device 308 also communicates with the CPU 1102over the bus 1104, and may include a CMOS pixel array having exemplarypixel cells 100, constructed as discussed above. The processor-basedsystem 1100 also includes random access memory (RAM) 1110, and caninclude removable memory 1115, such as flash memory, which alsocommunicates with CPU 1102 over the bus 1104. Imaging device 308 may becombined with a processor, such as a CPU, digital signal processor, ormicroprocessor, with or without memory storage on a single integratedcircuit or on a different chip than the processor. Any of the memorystorage devices in the processor-based system 1100 could store softwarefor employing the above-described method.

The above description and drawings are only to be consideredillustrative of exemplary embodiments which achieve the features andadvantages of the invention. Modification of, and substitutions to,specific process conditions and structures can be made without departingfrom the spirit and scope of the invention. Accordingly, the inventionis not to be considered as being limited by the foregoing descriptionand drawings, but is only limited by the scope of the appended claims.

1. A method of forming an image sensor pixel structure, said methodcomprising: forming a first and a second doped region of a firstconductivity type in a substrate of a second conductivity type, saidfirst and second doped regions corresponding to charge storage regions;forming a third and a fourth doped region in said substrate such thatsaid first doped region is below said third doped region to form aphotosensor and said fourth doped region is at least partially betweensaid first and second doped regions to form a charge barrier betweensaid first and second doped regions; forming a gate structure over atleast a portion of said second and said fourth region such that saidgate structure is operative to lower said charge barrier and gatecharges from said first doped region to said second doped region. 2-34.(canceled)